The present invention relates in general to differential pressure transducers and in particular to a new and useful differential pressure transducer that relies upon a Fabry-Perot fiber optic sensor to measure displacements and calculate differential pressure on the basis of such measurements.
Differential pressure (dP) transducers now known in the art usually contain a displacement sensor coupled between two thin diaphragms. It is well known in the art that the coupling between the two diaphragms can be accomplished with a mechanical linkage or through hydraulic coupling or both together. The two coupled diaphragms perform a mechanical subtraction of the pressures on them and the sensor measures their net motion relative to the transducer body to determine differential pressure. It has been the practice to fill the volume between the diaphragms with a hydraulic fluid in order to prevent rupture of the thin diaphragms due to the high static pressures while maintaining sensitivity to differential pressure. When the static pressure in the process line is presented to one side of each diaphragm, the fill fluid is pressurized to the line pressure. Deflection stops are incorporated to prevent rupture of the diaphragms in the event only one side of the transducer is subjected to high static pressure.
If the boundary of the volume between the diaphragms, including the diaphragms, electrical feedthroughs and fill/bleed ports, is not totally sealed small leaks of fill fluid will occur and will cause unacceptable increases in response time, sensor output drift and transducer non-linearity with pressure. In some cases, these changes may not be readily detected when the transducer is in service because the transducer output may remain stable at constant dP. The leaking of fill fluid from these known dP transducers is a problem that is well known and documented.
FIG. 1 depicts a first configuration for the known differential pressure transducer described above. Differential pressure cell 110 includes two spaced diaphragms 122 connected to a housing by seals 114. The diaphragms 122 are mechanically coupled to one another by way of the sensor. Fill fluid 116 and a sensor 120 are contained between the diaphragms 122 as well as a sensor lead wire 112. Diaphragm stops 124 are employed outside of the diaphragms 122. The pressure across the diaphragms 122 is measured as described herein.
FIG. 2 illustrates a second configuration. Here, the fluid-filled differential pressure transducer has a welded assembly 210 including isolation diaphragms 231, sensing diaphragm 233, fill fluid 216, and lead wires, which are also the high- and low-pressure fill tubes 225. In this configuration, the mechanical linkage between the two pressure isolation diaphragms is a third diaphragm called a sensing diaphragm. The measured change in capacitance between the sensing diaphragm 233 and high and low pressure metallized surfaces 237 is directly proportional to the pressure difference across the transducer 210. The cell further comprises process chambers 229 spaced away from convoluted pressure plates 235. Electrical insulator 221 is also provided as well as ceramic inserts 228.
The fill tube penetrations 225 in FIG. 2 are the locations most likely to leak fill fluid over time. In process plant pipeline application for example, the pressure on the fill fluid 216 is nominally 3000 psi at operating pressure. If the fill tube diameters 225 are kept small, the force acting on the fluid 216 to push it out of the fill tube penetrations 225 will also be small. Nevertheless, a good seal is difficult to maintain. Fill fluid leaks are also possible between the outside diameter of the fill tube 225 and the glass insulating material 221, and along the boundaries between the insulating material 221 and metal housing 211. Thus, it is very difficult to make a totally leak-tight seal, since penetrations through the high pressure boundary must be made. Similar leakage problems can occur with respect to the design of FIG. 1.
Another inadequacy of fluid-filled dP transducers is the static pressure effect. A dP transducer as described should output a value of zero when the same process pressure is applied to both diaphragms. However, the static pressure causes the fill fluid to be pressurized resulting in distortions of the transducer body. These distortions cause relative motions between the diaphragms and body resulting in static pressure effect on zero, and also produce radial forces on the diaphragms, which change their effective stiffness and cause static pressure effects on span. In addition, the displacement sensor is exposed to the fill-fluid pressure environment adding to the static pressure effects on both zero and span. In applications involving static pressures of about one thousand psig or greater, the requirement for a stable zero and span over the allowable range of static pressure is difficult to achieve in practice.
Yet another inadequacy of fluid-filled dP transducers is the effect of hydrogen. When dP transducers are operated in a hydrogen-rich environment, for example, in a hydrocarbon processing facility, the hydrogen gas easily diffuses through thin diaphragms and into the fill fluid. If the dP transducer is used to measure pressure differences such as may exist in a high pressure pipeline, then the fill fluid will experience the large static pipeline pressure. If hydrogen has diffused into the fill fluid, then when the pipeline pressure is reduced such as during a scheduled shutdown, the hydrogen gas boils out of the fill fluid and forms a bubble. Since the enclosed volume of fill fluid is constant, a bubble of hydrogen within the closed volume deforms the diaphragms, which results in a calibration shift, zero offset or in the worst case, diaphragm rupture. Such failures are normally undetected until the system is brought back on line resulting in safety issues and delays in production.
The use of a fill fluid also contributes to degraded performance of a dP transducer when it is operated over a range of temperatures, as is normal in service. The volumetric expansion of liquids with temperature is significantly greater than that of the metals used in construction of the transducer body. Thus, when the temperature of the transducer changes, the volume of the fill fluid changes more than the volume of the body. This results in motion of both the thin diaphragms away from their rest positions, distorting their shape and causing degraded linearity and accuracy. The normal method for limiting this effect is to keep the volume of the fill fluid at an absolute minimum; this method aggravates the effect of leakage because a leak of a given volume is a more significant part of the total fluid volume.
Rather than perform a mechanical subtraction of two large pressures as described above, an alternative approach would be to measure each pressure with a separate gage pressure transducers that do not require fill fluids. The differential pressure can be determined by subtracting the signals electronically. If the full-scale differential pressure range to be measured is 400 in H2O(15 psi) and the desired accuracy is 0.1% of Full Scale Range (0.015 psi), then for application at 3000 psig line pressure, a gage pressure transducer is required which has an accuracy of 0.015/3000=0.0005% (1:200,000). Such devices are not commercially available, and not yet achieved with any known technology. Thus, mechanical subtraction of two large pressures is the only alternative measurement approach available with present day technology.
U.S. Pat. No. 5,386,729 discloses a differential pressure transducer that does not require any fill fluid and uses a microbend fiber optic sensor which must be mounted to both the beam and the transducer housing. A microbend fiber optic sensor produces changes in light intensity that are detected by a detector to sense movement of a beam, e.g. a mechanical linkage, positioned between two pressure sensitive diaphragms. Backing stops are provided to prevent unwanted rupture of the diaphragms. Because the sensor relies upon intensity measurements which are themselves affected by movement of the beam or the transducer housing, the sensor can be affected by vibrations in either of the items to which it is mounted.
While this approach eliminates the need for fill-fluid, the reliance on intensity measurement by the fiber optic sensor leads to potential errors in light attenuation measurement by causes other than beam motion, e.g. drift in light source output, changes in losses in the optical fiber, couplers and connectors may all change the light intensity as well. Additionally, the beam and diaphragm must provide enough force to move the microbend sensor plates which are attached to the beam. This requires that the beam and diaphragms be relatively massive, which in turn means the overall assembly must be large and massive. Consequently, a dP cell with microbend sensors is subject to vibration sensitivity, as mentioned above, as well as thermal sensitivity and the costs associated with manufacturing such a dense and massive item. All of the shortcomings with microbend sensors are alleviated with an optical non-contact displacement sensor such as the Fabry-Perot fiber optic sensor which does not have to be positioned on the beam to measure the absolute position of the beam and is not sensitive to light intensity changes.
U.S. Pat. No. 6,425,290 and related U.S. Pat. No. 6,612,174 also contemplate differential pressure transducers that require the use of a capacitive sensors or diffraction gratings and interference patterns. Moreover, these differential pressure transducers do not contemplate the use of interferometric fiber optic Fabry-Perot sensors, nor do these sensors suggest a means of integrating a structural support and stopping mechanism directly into the sensor design.
Differential pressure transducers find utility in the measure of flow and level in applications where fluids are present and/or being transported, in addition to a myriad of other uses known to those skilled in the art. On occasion, such pressure transducers may be used in conjunction with orifice plates and venturi tubes for flow measurement in pipes. For the sake of simplicity, as used throughout this application and the appended claims, it should be understood the inventors use the terms orifice plates and venturi tubes interchangeably based upon their sumilar functionality.
Basically, the orifice plate is a round plate with a hole in the center. The plate serves as an obstruction to the flow. Capillary lines (i.e. tube) or impulse lines are connected to pressure taps in the pipe or on the flange at the location of the orifice plate. These lines are then connected to the differential pressure transducer and used to measure the pressure on each side of the orifice plate. Since the inner diameter of the pipe is much larger than the hole in the orifice plate, gas or liquid flowing in the pipe is forced through the smaller diameter orifice and the pressure on the upstream side is always greater than the pressure on the downstream side. The measured pressure difference across the orifice is proportional to the square of the mass flow rate and is normally measured in inches of water of differential pressure.
Presently, there is no known system or method for providing a differential pressure transducer which avoids the aforementioned problems by incorporating a Fabry-Perot interferometric fiber optic sensor. A differential pressure transducer which is relatively lightweight, easy to manufacture, immune to leakage of fill fluid, tolerant to very high temperatures, insensitive to thermal variations or vibrations, and immune to hydrogen migration would be welcomed by industry. A transducer that can be directly integrated with an orifice plate or venturi tube would also be welcomed.